There has been known a classifying method in which a liquid auxiliary agent such as an alcohol is previously added when a powder such as glassy blast furnace slag is classified into a fine powder and a coarse powder (for example, see Patent Literature 1). In this classifying method, an auxiliary agent containing polar molecules is added to the powder to electrically neutralize the polarity of powder particles, and, thus, to prevent formation of aggregated particles with a large grain size due to adsorption and aggregation of the particles, whereby a decrease in classification efficiency is prevented.